NOTE: This issue is raised due to confusion in #1139.

This issue is only about anonymous functions:

const identity = function (value) {
  //                     ^ space here
  return value;
}

(Prettier currently does not put a space there.)

🗒 NOTE: This issue is not about having a space after the function name:

function identity (value) {
  //             ^ space here: SEE #3845!
  return value;
}

Space after function name is tracked in #3845!

The key argument is:
Consistency: This means that there's always a space after the function keyword.